In the construction industry, it is frequently necessary to lay a large area of concrete. Such areas can include, for example, foundations for buildings, floors, driveways, sidewalks, ramps, etc. Concrete exhibits characteristics of strength in compression but is poor in tension. To increase strength in tension, it is common practice to prepare a grid of reinforcing bars and then to pour concrete over and around the grid whereby the reinforcing bars improve the strength of the poured concrete.
Although the quality of the concrete improves with the dryness of the mix, it is nevertheless axiomatic that pourable concrete is wet in nature. Concrete is a mixture of cement, sand and stones. Lime is an ingredient in cement and water is added to the mixture to form a mix or slurry. The water activates the lime and an insufficient amount of water will not activate the lime. Consequently, the reinforcing bars in the grid are surrounded with a wet product and, if unprotected, the bars will rust. While moisture alone will create an environment which is conducive to rust, the problem is exacerbated in situations where water becomes contaminated with salt(s). Such conditions can occur, for example, when roads are spread with salt in wintertime or when the concrete is poured in an oceanfront community.
When a steel bar rusts it expands, and it will be appreciated that expansion of the bar within the concrete will cause the set concrete to crack.
To overcome this problem, it has been practice in the art to envelop the reinforcing bars in a plastic-like coating, most commonly an epoxy, which will protect the bar from wet liquid and hence avoid rusting and the subsequent detrimental consequences thereof.
After the wet concrete has been poured over and around the grid of reinforcing bars, it is common practice in the art to vibrate the concrete to remove air and voids from the poured mix. In this manner, when the concrete hardens, the slab will be more compact and undesirable pockets within the hardened concrete are avoided and the integrity of the concrete is not compromised.
Concrete vibrators take the form of a metal cylinder with a rotator inside which causes the metal cylinder to vibrate. The rotator is mounted on one end of a flexible drive cable and the vibrating cylinder is introduced into, and immersed in, the wet concrete mix and generates vibrations therewithin. Such vibrations may be of a frequency of 10,000 vibrations per minute and an immediate problem arises in that such a rapidly vibrating metal cylinder will chip the plastic covering of the reinforcing bars, thereby exposing the metallic bars to the wet concrete and creating the potential for the destructive rust condition as described above.
To minimize this problem, advanced vibrator heads have been provided in which the metal cylinder is encased within a sleeve or sheath of robust but softer material. Such sheathed vibrators are commonly referred to as xe2x80x9crubberheadsxe2x80x9d. The advantage is immediately apparent in that, although the exterior of a rubberhead vibrates at the same frequency as the enclosed steel core, the relativexe2x80x9csoftnessxe2x80x9d of the sheath is less likely to chip the plastic coating of the reinforcing bars.
Although achieving the foregoing advantage of minimizing destruction as reinforcing bars, sheathed vibrators (xe2x80x9crubberheadsxe2x80x9d) create a disadvantage in that the steel core of the vibrator gets hot when vibrating at operating frequencies. With an unsheathed metal vibrator (xe2x80x9csteelheadxe2x80x9d) the heat generated can easily be dissipated in the wet concrete slurry. However, when the steel core is sheathed in the manner of a rubberhead, it is necessary to make provision for conducting heat away during operation. For this purpose, it is known to provide apertures or openings in the surrounding sheath which expose the inner metallic core and enable the wet concrete slurry to contact the core and conduct away the heat. If no such apertures or openings are provided, then a rubber coating completely encompassing and concealing the vibrating metallic core would serve as an insulating surface and would retain heat.
U.S. Pat. No. 5,556,199, owned by the applicant of this application, describes a rubberhead and the entire content of that earlier patent is incorporated herein by reference.
The present invention realizes that such apertures or openings passing through the sheath additionally function as suction cups in a rapidly vibrating vibrator and thereby hold wet concrete tight against the vibrating metallic core. The retention of concrete within the pockets by means of suction will cause the concrete to vibrate and thereby enhance the removal of air and elimination of pockets.
Although the suction cup effect of the apertures enhances vibration, the present invention greatly increases vibration by configuring the sheath to amplify the shockwaves transmitted into the wet concrete by the vibrating vibrator.
In prior vibrators, the cooling apertures or pockets in the sheath have been either straight-sided or flared outwardly from the surface of the metallic core to the outer surface of the sheath. With each of these configurations of the cooling apertures the apertures open into a relatively smooth surface portion of the sheath.
One embodiment of a vibrator of the present invention is cylindrical with a conical leading end and a tapered rearward end. A cylindrical steel core housing a vibrating mechanism is surrounded by a sheath of material which is softer than the metallic core. A plurality of apertures extend through the shaft to expose portions of the metallic core. The apertures are elongated and groups of apertures are disposed around the periphery of the sheath with the elongated apertures in each group having their longitudinal axes aligned with the longitudinal axis of the vibrator.
In order to increase the shockwaves imparted by the vibrating vibrator, the present invention configures the external sheath so that there are upstanding wall portions, at least around the apertures, which serve to xe2x80x9cdrivexe2x80x9d the shockwaves into the concrete surrounding the vibrator.
In order to provide an upstanding wall portion at the location of each cooling aperture, the configuration of the sheath is such that the leading side (in the direction of oscillation) of each elongated aperture is cut away in such manner that the cut away portion reduces the height of said leading side of the aperture. The cut away portion may lead directly to the steel core or there may be a step down from the level at which the cut away portion enters the aperture down to the steel core. With this configuration, the opposite or trailing side of the aperture is exposed and presents an upstanding wall portion which serves to xe2x80x9cdrivexe2x80x9d shockwaves into the concrete.
In addition to exposing the trailing side of the apertures in the manner described in the preceding paragraph, longitudinal recesses may be provided in the surface of the sleeve similarly to present an upstanding wall portions facing the direction of vibration.